Force the centre of rotation of a physical object to be the root prim centre?

[Profaitchikenz Haiku]

I came up against something I've not seen before:

Scripting a pendulum where the root prim is a sphere, attached to that is a 5-metre rod, attached to the end of that is a cylinder intended to be the weight. So a 3-prim linkset.

When non-physical, rotating the object around the root Y axis using KFM works as expected.

When physical, trying to rotate the object around the centre of the root prim using llRotLookAt , swings appear to be operating around what is either the geometric centre or the centre of mass, about midway along the rod.

I can find no way to force the centre of rotation to be the centre of the root prim. I'm not sure if I've overlooked something fundamental in physical objects or if this is something vehicle builders have always known about and I've missed because of living in an ivory tower?

 

ETA, OK, I forgot all about the object dimensions field that lets you fiddle around with what each object is made of, so it appears to be centre of mass that determines the rotation centre, not geometric centre.

Edited December 29, 2021 by Profaitchikenz Haiku

[Qie Niangao]

I was going to suggest changing the child links to Physics Type "None" which I suppose would have the same effect (although I haven't tried).

[Profaitchikenz Haiku]

I was hoping to adjust both the length of the rod and the size (and therefore mass) of the weight.

My aim is to try and see if there is a Foucalt's pendulum effect in SecondLiife. If there isn't, it would be a smart answer to some f the bozos who persist in the idea that the universe is a giant computer simulation.

[animats]

Pendulum in Builder's Brewery sandbox

The bearing is a knife-edge against a shallow V of knife edges. All knife edges have zero friction.

You can build one, and it will swing for a while, but not for very long. It doesn't slow down, it just stops at some random angle. That's something the physics engine does after a while so you can't have endlessly spinning wheels using up simulator time. Common in game engines.

It's quite possible to do a Focault pendulum in a physics system, but SL's is throttled down for overhead reasons.

 

 

[Profaitchikenz Haiku]

51 minutes ago, animats said:

You can build one, and it will swing for a while, but not for very long. It doesn't slow down, it just stops at some random angle.

I had got that far after making the root prim significantly larger and tweaking the dimensions of the rod and weight. The random angle stop had been driving me nuts and I was looking at whether I had to add energy to the system at each end of the swing. Thanks for the insight.